Soil order and grazing management effects on changes in soil C and N in New Zealand pastures

Careful temporal measurements of soil C and N stocks in pasture-based agriculture are needed to quantify effects of different grazing management practices on stocks for a wide range of soils. In a previous New Zealand study, 83 soil profiles were re-sampled two to four decades after an initial sampling. Soil C and N stocks were compared to that determined from archived soil samples from the initial sampling and indicated that there had been substantial losses of soil C and N under dairy grazing (about −0.73 Mg C ha−1 y−1 and −57 kg N ha−1 y−1 in top 0.3 m) but not from drystock grazing on flat land. Soil C and N increased for drystock grazing hill country (0.52 Mg C ha−1 y−1 and 66 kg N ha−1 y−1 in top 0.3 m). Here, we extended this re-sampling of soil profiles to 148 profiles to better balance the distribution of the major soil orders of New Zealand grazing land. We found that losses in soil C (0-0.3 m) between samplings were constrained to Allophanic (−1.37 kg C m−2) and Gley (−0.78 kg C m−2) Soils, but did not change for other soil orders for either drystock or dairy grazed pastures. Soil C also decreased between samplings in the 0.3-0.6 m depths for Allophanic Soils by −0.70 kg m−2. Soil N for Allophanic Soils decreased by −0.10 kg N m−2 (0-0.3 m) and by −0.068 kg N m−2 (0.3-0.6 m). In hill country pastures, 1.35 kg C m−2 and 0.16 kg N m−2 of soil C and N accumulated in the top 0.3 m, respectively, but there were insufficient samples to determine soil order effects. We suggest that losses of soil C from Gley Soils might be due to past drainage to improve production which accelerated organic matter decomposition. Losses from Allophanic Soils were unexpected but suggest C and N in these soils with large C and N stocks may be more vulnerable to losses than anticipated. Gains in hill country are likely due to regeneration of topsoil after about 70-150 years of widespread erosion following land conversion from forest to pasture. Except for Allophanic Soils, there was a significant decline in C:N ratio for both flat land and hill country demonstrating on-going N saturation of New Zealand pastures soils that will have consequences on N cycling and possible leakages to the N cascade. Because pastures on hill country cover a much larger land area than flat land (and particularly Allophanic and Gley Soils), we estimate a highly uncertain net increase in soil C at a national scale for the sampled soil orders.